In general, an engine control unit for controlling the engine of a vehicle operates with operating power supplied from the battery of the vehicle when the ignition switch of the vehicle is turned on. In recent years, an engine control unit is required to execute specific control processes at certain times even while the engine is not operating with the ignition switch turned off.
For instance, the specific control processes include an evaporation gas purge diagnosis process for detecting the leakage in an evaporation gas purge system.
The evaporation gas purge system is for purging evaporated fuel gas, which prevents the gaseous fuel evaporated in the fuel tank of a vehicle from being released into the atmosphere. The evaporation gas purge system makes the adsorbent in its canister temporarily adsorb the gaseous fuel, purges the adsorbed gaseous fuel into the intake pipe of the engine of the vehicle together with the fresh air sucked through the air hole of the canister in accordance with the operating condition of the engine, and burns the purged gaseous fuel (for instance, patent documents 1 and 2).
Patent Document 1: U.S. Pat. No. 5,575,265 (JP 1996-35452A)
Patent Document 2: JP P2001-173523A
If there are holes or cracks in the fuel tank, the evaporation passage between the tank and the canister, etc. of the evaporation gas purge system, the evaporated gaseous fuel is released into the atmosphere, without being adsorbed by the canister.
In order to prevent the air pollution caused by such failures of the evaporation gas purge system, the engine control unit executes an evaporation gas purging diagnosis process to detect the leakage in this system. The evaporation gas purging diagnosis process may involve checking the air tightness of the evaporation gas purge system by measuring the pressure fluctuation in the system by means of a pressure sensor, with the system blocked by a solenoid valve (for example, patent document 1).
In the evaporation gas purging diagnosis process, however, it is difficult to obtain accurate test results because the fuel in the fuel tank easily evaporates after the engine operates under a high load for a long time. Therefore, it is likely that the engine control unit executes the evaporation gas purging diagnosis process as a specific control process when a predetermined time has passed after the engine stops.
In order to meet the above requirement, the engine control unit may be kept operating with the operating power supplied even when the engine is not operating. In this case, however, the battery runs down because of high power consumption.
Therefore, in order to meet the above requirement with low power consumption, it is conceivable that the engine control unit is activated with the operating power supplied if the ignition switch is turned on, or if other power supply starting condition for executing the specific control process is met even when the ignition switch is maintained in the turned-off condition.
However, when the engine control unit is activated on the starting condition for executing the specific control process, this unit may drive electric loads unrelated to the specific control process. In this case also, the battery may run down because the engine is not operating and the battery is not cahrged.